The Applet is compiled to byte code by the developer, so that doesn't
affect the user startup time.  The JIT compiler is designed to be
somewhat transparent to the application user while increasing speed for
the user.

The compilation from bytecode to native code is an ongoing, background
process performed on code as it's run and profiled by the JVM.  Sun's
implementation ("Hotspot") is described in detail on their site:
 <http://java.sun.com/javase/technologies/hotspot/index.jsp>

In general JIT compilation trades a small slowdown initially (overhead
of running a lightweight profiler and the compilation process itself)
for a relatively large speedup over the life of the program.  I've
seen test cases for some things double in speed over a few minutes of
repeated tests, personally (for somewhat CPU-heavy operations: XML
parsing from a memory buffer).

You'll need to quantify your speed needs and measure for yourself
whether a given language is "fast enough", but personally I've found
Java to be more than fast enough for the vast majority of programs.

YMMV,
Owen

Will your application (SeeScreen?) be generating Java source code, then
(compiling and) executing it?

On a PC with a 1.7 GHz Celeron and 512MB RAM, running Windows XP Pro,
with many other applications open, it took about three seconds to
execute `javac HelloWorld.java`, it took about four seconds to compile a
 150 line Java class.

The JIT compilation happens in the background during execution
and should be invisible to the user.

Arne